This invention relates to oil and gas well casing hanger apparatus, and in particular to a mudline casing hanger assembly for supporting the casing at the mudline of a subsea well.
In such an apparatus commonly known as a mudline suspension system, a plurality of strings of casing are concentrically located, each one being supported within the successively outer one at the mudline. Each casing string which runs from the hanger downwardly is matched by an upward extension or conductor passing to the surface platform. Such a system is characterized by the severe limitation on annular space through which to run a hanger for support from the next adjacent hanger body.
The use of this annular space is further restricted by the need to obtain mud flow by area, both while running the hanger, and after the hanger has been landed. It is desirable that the hanger be capable of carrying high loads, and accordingly stress concentrations should be avoided. It is also desirable that the hanger be simply and reliably run.
One mudline hanger is illustrated in U.S. Pat. No. 4,139,059 issued to James T. Carmichael. This illustrates a split collet which is carried down with an inner hanger body by a ring. The collet latches to the outer hanger body at a preselected location, and the ring shears disengaging the collet from the inner body. Further downward movement of the inner body results on the body sitting on, and being supported by the collet.
In such a system the collet carries the entire load, and accordingly must be substantially sized. This provides limitations on available flow by. Since the support is through the collet on to the latching means the collet must be closely toleranced to assure approximately equal distribution of the load around the circumference. Furthermore, such a collet is continuously outwardly urged during the running operation and therefore the load bearing member, which is the collet, is dragged through the entire length of conductor from the surface to the support elevation.
An alternate mudline suspension system is shown in U.S. Pat. No. 3,497,243 issued to D. L. Gruller et al. This arrangement includes a plurality of downwardly extending fingers which are threadedly supported in a raised position within the outer hanger body. When the inner casing hanger body is to be supported, a tool is run down the string, rotating the apparatus to lower the downwardly extending fingers. These fingers are cammed in by the outer hanger body to provide a conical support surface for the later run inner hanger body.
The system requires two trips, one for moving the fingers, and the second for actually landing the casing hanger. The fingers are in place throughout the drilling operation which leads to possibility of wear, and damage to these load bearing surfaces. Furthermore, rotation of the drill string could cause premature energization of the mechanism driving the fingers down prematurely. These fingers also depend on a upwardly facing abutment to stop their movement. With the hanger body being in place during the drilling operation the potential exists for this abutment to become plugged or jammed such that the fingers override, and not engage the abutment. Modifications which could be made to assure engagement of the fingers with the abutment could result in even sharper corners at this high stress area, thereby leading to high stress concentrations.
Still another arrangement is illustrated in U.S. Pat. No. 4,355,825 issued to Leicht. This illustrates a C-shaped collet which rotates with the upper end moving outwardly to engage the collet latching means. Again, however, the entire load is carried through the collet resulting in size limitations for the highly loaded collet, and high stress concentrations at the sharp corners in the area of this support.